Reversibly-activated nanostructured battery

ABSTRACT

A battery having a nanostructured battery electrode is disclosed wherein it is possible to reverse the contact of the electrolyte with the battery electrode and, thus, to return a battery to a reserve state after it has been used to generate current. In order to achieve this reversibility, the nanostructures on the battery electrode comprise a plurality of closed cells and the pressure within the enclosed cells is varied. In a first embodiment, the pressure is varied by varying the temperature of a fluid within the cells by, for example, applying a voltage to electrodes disposed within said cells. In a second illustrative embodiment, once the battery has been fully discharged, the battery is recharged and then the electrolyte fluid is expelled from the cells in a way such that it is no longer in contact with the battery electrode.

FIELD OF THE INVENTION

The present invention relates generally to batteries and, moreparticularly, to batteries having nanostructured surfaces.

BACKGROUND OF THE INVENTION

Many beneficial devices or structures in myriad applications rely onbatteries as a power source. A typical liquid-cell battery, such asbattery 101 in FIG. 1, is characterized by an electrolyte liquid 102which provides a mechanism for an electrical charge to flow in direction103 between a positive electrode 104 and a negative electrode 105. Whensuch a battery 101 is inserted into an electrical circuit 106 withillustrative load 108, it completes a loop which allows electrons toflow in direction 107 around the circuit 106. The positive electrode 104thus receives electrons from the external circuit 106. These electronsthen react with the materials of the positive electrode 104 in reductionreactions that generate the flow of a charge to the negative electrode105 via ions in the electrolyte liquid 102. At the negative electrode105, oxidation reactions between the materials of the negative electrode104 and the charge flowing through the electrolyte fluid 102 result insurplus electrons that are released to the external circuit 106.

As the above process continues, the active materials of the positive andnegative electrodes 104 and 105, respectively, eventually becomedepleted and the reactions slow down until the battery is no longercapable of supplying electrons. At this point the battery is discharged.It is well known that, even when a liquid-cell battery is not insertedinto an electrical circuit, there is often a low level reaction with theelectrodes 104 and 105 that can eventually deplete the material of theelectrodes. Thus, a battery can become depleted over a period of timeeven when it is not in active use in an electrical circuit. This periodof time will vary depending on the electrolyte fluid used and thematerials of the electrodes.

More recently, batteries having at least one nanostructured surface havebeen proposed wherein nanostructures are used to separate theelectrolyte from the electrode until such a time that the battery is tobe used. This is typically referred to as a reserve battery (as opposedto a primary battery that is manufactured with the electrolyte incontact with the electrodes of the battery). An example of the use ofelectrowetting principles applied to reserve batteries is described incopending U.S. patent application Ser. No. 10/716,084 filed Nov. 18,2003 and entitled “Electrowetting Battery Having NanostructuredSurface,” which is hereby incorporated by reference herein in itsentirety. As disclosed in the '084 application, when it is desired thatthe battery generate a current, the electrolyte is caused to penetratethe nanostructured surface and to come into contact with the electrodeof the battery, thus resulting in the above-discussed flow of electronsaround a circuit. Such a penetration of nanostructures is achieved, forexample, by applying a voltage to the nanostructures such that thecontact angle of the electrolyte relative to the nanostructured surfaceis decreased. When the contact angle is decreased, the electrolytepenetrates the nanostructures and is brought into contact with theelectrode.

SUMMARY OF THE INVENTION

The present inventors have realized that, while prior reserve andprimary batteries were useful in many regards, they were limited incertain aspects. In particular, once the batteries were manufactured andactivated (in the case of a reserve battery), it was typicallyimpossible to return the batteries to a reserve state (i.e, to separatethe electrolyte from the battery electrodes).

Therefore, the present inventors have invented a small battery having ananostructured battery electrode wherein it is possible to reverse thecontact of the electrolyte with the battery electrode and, thus, toreturn a battery to a reserve state after it has been used to generatecurrent. In order to achieve this reversibility, the nanostructures onthe battery electrode comprise a plurality of closed cells and thepressure within the enclosed cells is varied. In a first embodiment, thepressure is varied by varying the temperature of a fluid within thecells by, for example, applying a voltage to electrodes disposed withinsaid cells. In a second illustrative embodiment, once the battery hasbeen fully discharged, the battery is recharged and then the electrolytefluid is expelled from the cells in a way such that it is no longer incontact with the battery electrode.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a prior art liquid-cell battery as used in an electricalcircuit;

FIG. 2 shows a prior art nanopost surface;

FIGS. 3A, 3B, 3C, 3D and 3E show various prior art nanostructure featurepatterns of predefined nanostructures that are suitable for use in thepresent invention;

FIG. 4 shows a more detailed view of the prior art nanostructure featurepattern of FIG. 3C;

FIGS. 5A and 5B show a device in accordance with the principles of thepresent invention whereby electrowetting principles are used to cause aliquid droplet to penetrate a nanostructure feature pattern;

FIG. 6 shows the detail of an illustrative nanopost of the nanostructurefeature pattern of FIGS. 5A and 5B;

FIG. 7 shows an illustrative liquid-cell battery in accordance with theprinciples of the present invention wherein the electrolyte in thebattery is separated from the negative electrode by nanostructures;

FIG. 8 shows the illustrative battery of FIG. 7 wherein the electrolytein the battery is caused to penetrate the nanostructures and to thuscontact the negative electrode; and

FIGS. 9A, 9B and 9C show a battery with the principles of the presentinvention wherein a droplet of electrolyte is disposed in an initialposition suspended on top of a nanostructured feature pattern (FIG. 9A),is caused to penetrate the feature pattern (FIG. 9B), and is then causedto return to a position suspended on top of the feature pattern (FIG.9C);

FIGS. 10A and 10B show an illustrative closed-cell structure inaccordance with the principles of the present invention;

FIGS. 11A and 11B show the detail of one cell in the illustrativestructure of FIGS. 10A and 10B; and

FIGS. 12A, 12B and 12C show a battery in accordance with the principlesof the present invention wherein a droplet of electrolyte is disposed inan initial position suspended on top of a nanostructured feature pattern(FIG. 12A), is caused to penetrate the feature pattern (FIG. 12B), andis then caused to return to a position suspended on top of the featurepattern (FIG. 12C).

DETAILED DESCRIPTION

FIG. 2 shows an illustrative nanopost pattern 201 with each nanopost 209having a diameter of less than 1 micrometer. While FIG. 2 showsnanoposts 209 formed in a somewhat conical shape, other shapes and sizesare also achievable. In fact, cylindrical nanopost arrays have beenproduced with each nanopost having a diameter of less than 10 nm.Specifically, FIGS. 3A-3E show different illustrative arrangements ofnanoposts produced using various methods and further show that suchvarious diameter nanoposts can be fashioned with different degrees ofregularity. Moreover, these figures show that it is possible to producenanoposts having various diameters separated by various distances. Anillustrative method of producing nanoposts, found in U.S. Pat. No.6,185,961, titled “Nanopost arrays and process for making same,” issuedFeb. 13, 2001 to Tonucci, et al, is hereby incorporated by referenceherein in its entirety. Nanoposts have been manufactured by variousmethods, such as by using a template to form the posts, by various meansof lithography, and by various methods of etching.

FIG. 4 shows the illustrative known surface 401 of FIG. 3C with ananostructure feature pattern of nanoposts 402 disposed on a substrate.Throughout the description herein, one skilled in the art will recognizethat the same principles applied to the use of nanoposts ornanostructures can be equally applied to microposts or other largerfeatures in a feature pattern. The surface 401 and the nanoposts 402 ofFIG. 4 are, illustratively, made from silicon. The nanoposts 402 of FIG.4 are illustratively approximately 350 nm in diameter, approximately 6μm high and are spaced approximately 4 μm apart, center to center. Itwill be obvious to one skilled in the art that such arrays may beproduced with regular spacing or, alternatively, with irregular spacing.

As typically defined a “nanostructure” is a predefined structure havingat least one dimension of less than one micrometer and a“microstructure” is a predefined structure having at least one dimensionof less than one millimeter. However, although the disclosed embodimentsrefer to nanostructures and nanostructured surfaces, it is intended bythe present inventors, and will be clear to those skilled in the art,that microstructures may be substituted in many cases. Accordingly, thepresent inventors hereby define nanostructures to include bothstructures that have at least one dimension of less than one micrometeras well as those structures having at least one dimension less than onemillimeter. The term “feature pattern” refers to either a pattern ofmicrostructures or a pattern of nanostructures. Further, the terms“liquid,” “droplet,” and “liquid droplet” are used hereininterchangeably. Each of those terms refers to a liquid or a portion ofliquid, whether in droplet form or not.

In many applications, it is desirable to be able to control thepenetration of a given liquid into a given nanostructured ormicrostructured surface and, thus, control the contact of the liquidwith the underlying substrate supporting the nanostructures ormicrostructures. FIGS. 5A and 5B show one embodiment whereelectrowetting is used to control the penetration of a liquid into ananostructured surface. Electrowetting principles and controlling themovement of a liquid across a nanostructured or microstructured surfaceare generally described in U.S. patent application Ser. No. 10/403,159filed Mar. 31, 2003 and titled “Method And Apparatus For VariablyControlling The Movement Of A Liquid On A Nanostructured Surface,” whichis hereby incorporated by reference herein in its entirety. As discussedpreviously, the general use of electrowetting principles in batteries isdescribed in above-referenced copending U.S. patent application Ser. No.10/716,084.

Referring to FIG. 5A, a droplet 501 of conducting liquid (such as anelectrolyte solution in a liquid-cell battery) is disposed onnanostructure feature pattern of cylindrical nanoposts 502, as describedabove, such that the surface tension of the droplet 501 results in thedroplet being suspended on the upper portion of the nanoposts 502. Inthis arrangement, the droplet only covers surface area f₁ of eachnanopost and has a contact angle with each nanopost of, for example, θ₀.The nanoposts 502 are supported by the surface of a conducting substrate503. Droplet 501 is illustratively electrically connected to substrate503 via lead 504 having voltage source 505. An illustrative nanopost isshown in greater detail in FIG. 6. In that figure, nanopost 502 iselectrically insulated from the liquid (501 in FIG. 5A) by material 601,such as an insulating layer of dielectric material. The nanopost isfurther separated from the liquid by a low surface energy material 602,such as a well-known fluoro-polymer. Such a low surface energy materialallows one to obtain an appropriate initial contact angle (i.e., θ₀)between the liquid and the surface of the nanopost. It will be obviousto one skilled in the art that, instead of using two separate layers ofdifferent material, a single layer of material that possessessufficiently low surface energy and sufficiently high insulatingproperties could be used.

FIG. 5B shows that, by applying a low voltage (e.g., 10-20 volts) to theconducting droplet of liquid 501, a voltage difference results betweenthe liquid 501 and the nanoposts 502. The contact angle between theliquid and the surface of the nanopost decreases and, at a sufficientlylow contact angle, the droplet 501 moves down in the y-direction alongthe surface of the nanoposts 502 and penetrates the nanostructurefeature pattern until it completely surrounds each of the nanoposts 502and comes into contact with the upper surface of substrate 503. In thisconfiguration, the droplet covers surface area f₂ of each nanopost.Since f₂>>f₁, the overall contact area between the droplet 501 and thenanoposts 502 is relatively high such that the droplet 501 contacts thesubstrate 503. One skilled in the art will recognize that other methodsof causing the electrolyte to penetrate the nanostructures, such asdecreasing the temperature of the electrodes, can be used. The presentinvention is intended to encompass any such method of causing suchpenetration.

FIG. 7 shows an illustrative battery 701 whereby an electrolyte fluid702 is contained within a housing having containment walls 703. Theelectrolyte fluid 702 is in contact with positive electrode 704, but isseparated from negative electrode 708 by nanostructured surface 707.Nanostructured surface 707 may be the surface of the negative electrodeor, alternatively, may be a surface bonded to the negative electrode.One skilled in the art will recognize that the nanostructured surfacecould also be used in association with the positive electrode withsimilarly advantageous results. In FIG. 7, the electrolyte fluid issuspended on the tops of the nanoposts of the surface, similar to thedroplet of FIG. 5A. The battery 701 is inserted, for example, intoelectrical circuit 705 having load 706. When the electrolyte liquid 702is not in contact with the negative electrode, there is substantially noreaction between the electrolyte and the electrodes 704 and 708 of thebattery 701. Accordingly, there is no depletion of the materials of theelectrodes. Thus, it is possible to store the battery 701 for relativelylong periods of time without the battery becoming discharged.

FIG. 8 shows the battery 701 of FIG. 7 inserted into electrical circuit705 wherein, utilizing the electrowetting principles described above, avoltage is applied to the nanostructured surface 707 thus causing theelectrolyte fluid 702 to penetrate the surface 707 and to come intoelectrical contact with the negative electrode 708. One skilled in theart will recognize that this voltage can be generated from any number ofsources such as, for example, by passing one or more pulses of RF energythrough the battery. When the penetration of the electrolyte into thenanostructures occurs, electrons begin flowing in direction 801 throughthe circuit 705, as described above, and the load 706 is powered. Thus,the embodiment of FIGS. 7 and 8 show how a battery can be stored withoutdepletion for a relatively long period of time and can then be “turnedon” at a desired point in time to power one or more electrical loads inan electrical circuit.

The battery described in FIGS. 7 and 8 is referred to as a reservebattery or, in other words, a battery that is manufactured with theelectrolyte separated from at least one of the electrodes in thebattery. Primary batteries, on the other hand, are batteries that aremanufactured with the electrolyte in contact with the electrodes of thebattery. As such, primary batteries are always undergoing oxidationreactions, even when not inserted in an electrical circuit. Therefore,primary batteries typically have a relatively short shelf-life relativeto reserve batteries.

The present inventors have recognized that it would be desirable to beable to selectively turn on and off the generation of current in abattery. Such a capability would have many novel uses. For example, thebattery could be turned on only when it was needed, thus preventingexcess oxidation that could lead to premature discharge of the battery.Additionally, such a capability could lead to a new category of reserverechargeable batteries that, once recharged, can be turned off. As iswell-known, rechargeable batteries (also referred to herein as secondarybatteries) are batteries in which the electrodes can be regenerated byreversing the current flow to and within the battery. While it ispossible to recharge the reserve nanostructured batteries describedpreviously, no effective methods have yet been realized for returningthe recharged battery to a reserve state once it is recharged.

The present inventors have further realized that, in the nanostructuredbatteries discussed above herein, it would be desirable to reverse thepenetration of the electrolyte in a way such that it is restored to itsoriginal reserve position suspended on the nanostructures above theelectrode. Reversible penetration of nanostructured or microstructuredsurfaces by a droplet of liquid is the subject of copending U.S. patentapplication Ser. No. 10/674,448, filed Sep. 30, 2003 and entitled“Reversible Transitions On Dynamically Tunable Nanostructured OrMicrostructured Surfaces,” which is hereby incorporated by referenceherein in its entirety.

FIGS. 9A, 9B and 9C illustrate a selective/reversible penetration ofdroplet 901, which is illustratively a droplet of electrolyte such aselectrolyte 702 in FIG. 7 into nanostructure pattern 904. Specifically,FIG. 9A shows electrolyte droplet 901 disposed on a nanostructure ormicrostructure feature pattern 904 that is supported by substrate 905(which is, illustratively, the electrode 503 in FIG. 5A). Next, as shownin FIG. 9B and discussed above, droplet 901 is caused to penetrate thefeature pattern 904. Finally, as shown in FIG. 9C, it is desirable toreverse the penetration of droplet 902. FIGS. 10A and 10B show,respectively, a three-dimensional view and a top cross-sectional view ofan illustrative feature pattern in accordance with the principles of thepresent invention that is capable of accomplishing the reversiblepenetration shown in FIGS. 9A-9C. Specifically, in the presentillustrative embodiment represented by FIGS. 10A and 10B, the featurepattern does not comprise a number of posts spaced a distance away fromeach other. Instead, a number of closed cells 1001, here illustrativecells of a hexagonal cross section, are used. Each cell 1001 has anelectrode 1002 disposed along the inner wall of the cell. As usedherein, the term closed cell is defined as a cell that is enclosed onall sides except for the side upon which a liquid, such as anelectrolyte liquid, is intended to be disposed. One skilled in the artwill recognize that other, equally advantageous cell configurations andgeometries are possible to achieve equally effective closed-cellarrangements. FIGS. 11A and 11B show a top cross-sectional view and aside view of an illustrative individual cell of the feature pattern ofFIGS. 10A and 10B. Specifically, referring to FIG. 11A, each individualcell 1101 is characterized by a maximum width 1102 of width d, anindividual side length 1103 of length d/2 and a wall thickness 1104 ofthickness t. Referring to FIG. 11B, the height 1105 of cell 1101 isheight h.

FIGS. 12A, 12B and 12C show how an illustrative closed-cell featurepattern similar to the feature pattern of FIGS. 10A and 10B, here shownin cross-section, may be used illustratively to cause a droplet 1201 ofliquid to reversibly penetrate the feature pattern. Specifically, eachcell within feature pattern 1204, such as cell 1101 having a hexagonalcross-section, is a completely closed cell once the droplet of liquidcovers the opening of that cell. Thus, referring to FIG. 12A, each suchclosed cell over which the droplet is disposed contains a fluid havingan initial temperature T=T₀ and an initial pressure P=P₀. As usedherein, the term fluid is intended to encompass both gases (such as,illustratively, air) and liquids that could be disposed within the cellsof the feature pattern. The present inventors have recognized that, bychanging the pressure within the individual cells, such as cell 1101,the liquid droplet 1201 can be either drawn into the cells or,alternatively, repelled out of the cell. Specifically, referring to FIG.12B, if the pressure within the cell 1101 is caused to be below theinitial pressure (i.e., P<P₀), then the droplet above that cell will bedrawn into the cell a distance related to the magnitude in reduction ofthe pressure P. Such a reduction in pressure may be achieved,illustratively, by reducing the temperature of the fluid within thecells such that T<T₀. Such a temperature reduction may be achieved,illustratively, by reducing the temperature of the substrate 1205 and/orthe feature pattern 1204. One skilled in the art will recognize that anymethod of reducing the pressure within the cells, including any othermethod of reducing the temperature of the fluid within the cells, willhave similar results. For example, each of the cells could be connectedeither in series or in parallel to one or more remote ballast gasreservoirs. The pressure of the gas in this reservoir could be changed,thus raising or lowering the pressure in the cells. Similarly, thepressure within the cells could be changed by moving a diaphragmdisposed within each of the cells, thus displacing a fluid within thecell and varying the pressure within that cell. Additionally, asdiscussed more fully in the aforementioned copending patentapplications, electrowetting may be used instead of pressure reductionto draw the liquid into the cells of the feature pattern 1204.Specifically, by applying a voltage to the conducting drople 1201, avoltage difference results between the liquid and the cells in thefeature pattern 1204. Hence, as discussed herein above, the droplet 1201moves down and penetrates the nanostructure feature pattern 1204 untilit comes into contact with the upper surface of substrate 1205. Othermethods of changing the pressure within the cells will be readilyapparent to one skilled in the art in light of the teachings herein.

FIG. 12C shows how, by increasing the pressure to or above the initialpressure P₀, it is possible to reverse the penetration of the droplet1201, whether that penetration was initiated by pressure reduction or byelectrowetting. Once again, such a pressure increase may be achieved bychanging the temperature of the fluid within the cells, illustrativelyin FIG. 12C to a temperature greater than the initial temperature T₀.One illustrative method if increasing this temperature is to apply avoltage to electrodes 1002 in FIG. 10 in a way such they heat theinsides of the cells. The increased temperature will increase thepressure within the cells above the initial pressure P₀. The contactangle between the droplet and the elements of the feature pattern willthus change to θ₃, which is smaller than θ₁ and the liquid will move outof the cells, thus returning droplet 1201 to a very low flow resistancecontact with feature pattern 1204. Once again, one skilled in the artwill recognize that any method of increasing the pressure within thecells to reverse the penetration of the droplet 1201, including anyother method of increasing the temperature of the fluid within thecells, will have similar results.

The foregoing merely illustrates the principles of the invention. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the invention and are within itsspirit and scope. For example, one skilled in the art, in light of thedescriptions of the various embodiments herein, will recognize that theprinciples of the present invention may be utilized in widely disparatefields and applications. For example, while the embodiment disclosedherein is a battery having nanostructured surfaces, one skilled in theart will appreciate that such nanostructured surfaces may be used forother uses, such as in use as a thermostat. In such a case, thecharacteristics of the pattern of nanostructures and the liquid incontact with the nanostructures can be chosen in a way such that, upon atemperature increase of known amount, the liquid will penetrate thesurface, thus achieving a desired result. One skilled in the art will beable to devise many similar uses of the underlying principles associatedwith the present invention, all of which are intended to be encompassedherein. All examples and conditional language recited herein areintended expressly to be only for pedagogical purposes to aid the readerin understanding the principles of the invention and are to be construedas being without limitation to such specifically recited examples andconditions. Moreover, all statements herein reciting aspects andembodiments of the invention, as well as specific examples thereof, areintended to encompass functional equivalents thereof.

1. A battery comprising: an electrode having at least one surface; aplurality of nanostructures, disposed on said surface to form a featurepattern on said surface; an electrolyte liquid in contact with saidnanostructures, and a fluid disposed within said feature pattern,wherein said fluid is controllable to reversibly change a degree ofpenetration of said feature pattern by said electrolyte liquid.
 2. Thebattery of claim 1 wherein said nano structures are closed cells.
 3. Thebattery of claim 1 wherein a temperature of said fluid is increased ordecreased to cause said change.
 4. The battery of claim 1 wherein saidfeature pattern comprises a plurality of posts.
 5. The battery of claim1 wherein said feature pattern comprises a plurality of closed cells. 6.The battery of claim 2 or 5 further comprising one or more cellelectrodes disposed within at least a portion of said closed cells,wherein, upon causing electrical current to flow through said cellelectrodes, a temperature of said fluid increases to reduce said degreeof penetration.
 7. The battery of claim 2 or 5 wherein said closed cellshave a hexagonal cross section.
 8. The battery of claim 1 wherein saiddegree of penetration is controlled by changing a pressure of said fluiddisposed within a closed cell of said feature pattern.
 9. The battery ofclaim 1 wherein said fluid is a gas.
 10. The battery of claim 1 whereinsaid degree of penetration is controlled by changing a contact angleformed between said electrolyte liquid and said nano structures.
 11. Thebattery of claim 1, wherein said fluid is said electrolyte liquid. 12.The battery of claim 1, wherein said fluid is a gas.
 13. The battery ofclaim 1, wherein reducing a temperature of said electrode causes saiddegree of penetration to increase.